Proton exchange membrane fuel cells (PEMFC) have attracted considerable attention as promising power generators for automotive, stationary, as well as portable power, due to their high-energy efficiency and low emissions. The membrane is one of the key components in the design of improved polymer electrolyte membrane fuel cells. It has three main functions as electrolyte medium for ion conduction and electrode reactions, as a barrier for separating reactant gases, and as the support for electrode catalysts. An applicable PEMFC membrane should possess high ionic conductivity, low electronic conductivity, good chemical, thermal and oxidative stability as well good mechanical properties. Current technologies are based on sulfonated membranes, such as Nafion, although it is not suitable at high temperatures or under low relative humidity conditions. Also, its methanol crossover and high cost have still to be overcome for commercialization. Current research on PEMFCs is focused on the optimization of a device working at operational temperatures above 100° C. and at very low humidity levels. Operation of the fuel cells at elevated temperatures has the benefits of reducing CO poisoning of the platinum electrocatalyst and increased reaction kinetics. In this respect, new polymeric materials have been synthesized in order to replace Nafion. One of the most successful high temperature polymer membranes developed so far is the phosphoric acid-doped Polybenzimidazole (PBI). Apart from high thermal stability and good membrane-forming properties, PBI contains basic functional groups which can easily interacts with strong acids, such as H3PO4 and H2SO4, allowing proton migration along the anionic chains. Even though PBI membranes show high proton conductivity at high temperature (>100° C.) under low relative humidity conditions and have a high CO tolerance, they exhibit low oxidative stability and moderate mechanical properties. Beside Polybenzimidazole (PBI), there is a significant research effort nowadays towards the development of some novel polymeric materials, which fulfill the prerequisites for use in high temperature PEMFCs. Poly(2,5-benzimidazole) (ABPBI) is an alternative benzimidazole type polymer with thermal stability and conducting properties as good as those of PBI. On the other hand, high-temperature aromatic polyether type copolymers containing basic groups like PBI enable formation or complexes with stable acids and exhibit high thermal, chemical stability and good conducting properties in order to be used in high temperature PEMFCs.
Prior art related to methods of making membrane electrode assemblies covers issues in the following areas: (i) direct membrane catalyzation, (ii) catalyzation of coated electrode substrates, (iii) need for effecting membrane electrode bonding for seamless proton transport (iv) effective solubility of reactant gases (in particular oxygen), (v) use of pore forming agents for effective gas transport within the electrode structure. Prior art literature relates to the specific objective of enhancing mass transport and the ability to operate a fuel cell on a sustained higher power density level.
In the context of prior art, direct catalyzation of the membrane has been described in various patents and scientific literature primarily on aqueous based polymer electrolytes, most notably of the perfluorinated sulfonic acid type. At the current state of the technology, prior efforts together with current approaches have to be tempered with the ability to translate developments in this regard to mass manufacturability while keeping reproducibility (batch vs. continuous) and cost in perspective. Depending on the deposition methods used, the approach towards lowering noble metal loading can be classified into five broad categories, (i) thin film formation with carbon supported electrocatalysts, (ii) pulse electrodeposition of noble metals (Pt and Pt alloys), (iii) sputter deposition (iv) pulse laser deposition, and (v) ion-beam deposition. While the principal aim in all these efforts is to improve the charge transfer efficiency at the interface, it is important to note that while some of these approaches provide for a better interfacial contact allowing for efficient movement of ions, electrons and dissolved reactants in the reaction zone, others additionally effect modification of the electrocatalyst surface (such as those rendered via sputtering, electrodeposition or other deposition methods).
In the first of the five broad categories using the ‘thin film’ approach in conjunction with conventional carbon supported electrocatalysts, several variations have been reported, including (a) the so called ‘decal’ approach where the electrocatalyst layer is cast on a PTFE blank and then decaled on to the membrane (Wilson and Gottesfeld 1992; Chun, Kim et al. 1998). Alternatively an ‘ink’ comprising of Nafion® solution, water, glycerol and electrocatalyst is coated directly on to the membrane (in the Na+ form) (Wilson and Gottesfeld 1992). These catalyst coated membranes are subsequently dried (under vacuum, 160° C.) and ion exchanged to the H+ form (Wilson and Gottesfeld 1992). Modifications to this approach have been reported with variations to choice of solvents and heat treatment (Qi and Kaufman 2003; Xiong and Manthiram 2005) as well as choice of carbon supports with different microstructure (Uchida, Fukuoka et al. 1998). Other variations to the ‘thin film’ approach have also been reported such as those using variations in ionomer blends (Figueroa 2005), ink formulations (Yamafuku, Totsuka et al. 2004), spraying techniques (Mosdale, Wakizoe et al. 1994; Kumar and Parthasarathy 1998), pore forming agents (Shao, Yi et al. 2000), and various ion exchange processes (Tsumura, Hitomi et al. 2003). At its core this approach relies on extending the reaction zone further into the electrode structure away from the membrane, thereby providing for a more three dimensional zone for charge transfer. Most of the variations reported above thereby enable improved transport of ions, electrons and dissolved reactant and products in this ‘reaction layer’ motivated by need to improve electrocatalyst utilization. These attempts in conjunction with use of Pt alloy electrocatalysts have formed the bulk of the current state of the art in the PEM fuel cell technology. Among the limitations of this approach are problems with controlling the Pt particle size (with loading on carbon in excess of 40%), uniformity of deposition in large scale production and cost (due to several complex processes and/or steps involved).
An alternative method for enabling higher electrocatalyst utilization has been attempted with pulse electrodeposition. Taylor et al., (Taylor, Anderson et al. 1992) one of the first to report this approach used pulse electrodeposition with Pt salt solutions which relied on their diffusion through thin Nafion® films on carbon support enabling electrodeposition in regions of ionic and electronic contact on the electrode surface. See a recent review on this method by Taylor et al., describing various approaches to pulse electrodeposition of catalytic metals (Taylor and Inman 2000). In principal this methodology is similar to the ‘thin film’ approach described above, albeit with a more efficient electrocatalyst utilization, since the deposition of electrocatalysts theoretically happens at the most efficient contact zones for ionic and electronic pathways. Improvements to this approach have been reported such as by Antoine and Durand (Antoine and Durand 2001) and by Popov et al., (Popov 2004). Developments in the pulse algorithms and cell design have enabled narrow particle size range (2-4 nm) with high efficiency factors and mass activities for oxygen reduction. Though attractive, there are concerns on the scalability of this method for mass scale manufacturing.
Sputter deposition of metals on carbon gas diffusion media is another alternative approach. Here however, interfacial reaction zone is more in the front surface of the electrode at the interface with the membrane. The original approach in this case was to put a layer of sputter deposit on top of a regular Pt/C containing conventional gas diffusion electrode. Such an approach (Mukerjee, Srinivasan et al. 1993) exhibited a boost in performance by moving part of the interfacial reaction zone in the immediate vicinity of the membrane. Recently, Hirano et al. (Hirano, Kim et al. 1997) reported promising results with thin layer of sputter deposited Pt on wet proofed non catalyzed gas diffusion electrode (equivalent to 0.01 mgPt/cm2) with similar results as compared to a conventional Pt/C (0.4 mgPt/cm2) electrode obtained commercially. Later Cha and Lee (Cha and Lee 1999), have used an approach with multiple sputtered layers (5 nm layers) of Pt interspersed with Nafion®-carbon-isopropanol ink, (total loading equivalent of 0.043 mgPt/cm2) exhibiting equivalent performance to conventional commercial electrodes with 0.4 mgPt/cm2. Haug et al. (Haug 2002) studied the effect o substrate on the sputtered electrodes. Further, O'Hare et al., on a study of the sputter layer thickness has reported best results with a 10 nm thick layer. Further, significant advancements have been made with sputter deposition as applied to direct methanol fuel cells (DMFC) by Witham et al. (Witham, Chun et al. 2000; Witham, Valdez et al. 2001) wherein several fold enhancements in DMFC performance was reported compared to electrodes containing unsupported PtRu catalyst. Catalyst utilization of 2,300 mW/mg at a current density of 260 to 380 mA/cm2 was reported (Witham, Chun et al. 2000; Witham, Valdez et al. 2001). While the sputtering technique provides for a cheap direct deposition method, the principal drawback is the durability. In most cases the deposition has relatively poor adherence to the substrate and under variable conditions of load and temperature, there is a greater probability of dissolution and sintering of the deposits.
An alternative method dealing direct deposition was recently reported using pulsed laser deposition (Cunningham, Irissou et al. 2003). Excellent performance was reported with loading of 0.017 mgPt/cm2 in a PEMFC, however this was only with the anode electrodes, no cathode application has been reported to date.
However, in all these new direct deposition methodologies, mass manufacturability with adequate control on reproducibility remains questionable at best. In this regard the methodologies developed by 3 M company is noteworthy, where mass manufacture of electrodes with low noble metal loading is reported (Debe, Pham et al. 1999; Debe, Poirier et al. 1999). A series of vacuum deposition steps are involved with adequate selection of solvents and carbon blacks resulting in nanostructured noble metal containing carbon fibrils which are embedded into the ionomer-membrane interface (Debe, Haugen et al. 1999; Debe, Larson et al. 1999).
An alternative is the use of ion-beam techniques, where the benefits of low energy ion bombardment concurrent to thin film vacuum deposition (electron beam) process is exploited for achieving dense, adhering and robust depositions (Hirvonen 2004). This method has been recently reviewed (Hirvonen 2004) in terms of both mechanisms of ion/solid interactions during thin film growth as well as development of various protocols for specific application areas, including tribology, anti corrosion coatings, superconducting buffer layers and coatings on temperature sensitive substrates such as polymers. Modifications of this approach to prepare 3-D structures including overhang and hollow structures have also been recently reported (Hoshino, Watanabe et al. 2003). Use of dual anode ion source for high current ion beam applications has also been reported recently (Kotov 2004), where benefits for mass production environment is discussed.
It thus would be desirable to provide a method for improving the catalyst utilization at the interface of a polymer electrolyte imbibed with ion conducting components (such as phosphoric, polyphosphoric and analogs of perfluorinated sulfonic acids) so as to enable higher power densities (i.e., 400 mW/cm2 at 0.5 V vs. RHE, 170-180° C., H2/Air). It would also be desirable to provide improved power density attained with lower Pt loading (0.3 to 0.4 mg/cm2) as compared to the current state of the art which is in the range 0.5 to 1.0 mg/cm2, thus providing for a better gravimetric energy density. It would be further desirable to provide an improved ability to retain ion conducting elements (such as phosphoric, polyphosphoric and analogs of perfluorinated sulfonic acids) within the reaction layer (catalyst containing zone at the interface between the electrode and the membrane). It would be particularly desirable from the perspective of long term sustained power density as well as better tolerance to both load and thermal cycling (especially transitions to below the condensation zone).